1. Field of the Invention
The present invention relates to a photocatalyst based on apatite. More specifically, the present invention relates to a photocatalytic apatite composition capable of expressing an excellent antimicrobial property in a dark place (in a state that ultraviolet light is not available), and a production method thereof. The present invention also relates to an article using such a photocatalytic apatite composition. The photocatalytic apatite composition of the present invention can be advantageously used for articles such as filters for air cleaners or air conditioners and for wallpaper. These articles are particularly useful in places, where an antimicrobial property is required, such as hospital and healthcare centers.
2. Description of the Related Art
A semiconductor substance such as titanium oxide is known to provide a photocatalytic function. That is, when this semiconductor substance absorbs light energy at a wavelength corresponding to a gap between the valence band and the conduction band, an electron of the valence band transfers to the conduction band, as a result of the excitation, and a hole is generated in the valence band. In the conduction band, assuming that a certain substance (for example, an organic material) is adsorbed at the surface of the semiconductor substance, the electron transferred to this band moves to the organic material on the surface of the semiconductor, thereby reducing the organic material. Also, in the valence band, the hole generated therein captures an electron and thereby oxidizes the organic material. Particularly, in the case of titanium oxide, the hole in the valence band has strong oxidizing power and therefore, can decompose the organic material, finally, into water and carbon dioxide. By using such a photocatalytic function (oxidative decomposition function) of titanium oxide, a titanium oxide film is used as an antimicrobial agent, a germicide, a deodorant, an environment cleaning agent or the like. For example, International Patent Publication No. WO00/06300 describes a method for producing functional materials having photocatalytic functions, such as tiles and sanitary ware, wherein a photocatalyst coating composition comprising a photocatalytic metal oxide such as titanium oxide (TiO2) is coated and then, the substrate surface is rapidly heated to fix the photocatalytic metal oxide on the substrate surface.
In recent years, products where the photocatalyst is also used in the filters of air cleaners or air conditioners are emerging, and titanium oxide is also used as the photocatalyst for the production of such filters. For example, Japanese Unexamined Patent Publication (Kokai) No. 10-33921 describes a filter characterized in that an amorphous calcium phosphate (preferably tricalcium phosphate containing crystal water) and a photo-semiconductor composition (preferably titanium oxide, calcium sulfide or the like) are dispersed and contained in a sheet-like shaped substrate (preferably, paper, woven fabric, nonwoven fabric or plastic foam), or an amorphous calcium phosphate and a photo-semiconductor composition are bonded to such a substrate through an adhesive. However, titanium oxide itself has no adsorption ability and, therefore, in many filters, an adsorbent as represented by activated carbon is used in combination, but an organic material adsorbed on the activated carbon cannot be entirely decomposed by titanium oxide and only an organic material adsorbing to a portion where an activated carbon particle and a titanium oxide particle are present adjacently can be decomposed. Therefore, the decomposition efficiency is low.
On the other hand, it is proposed to use, in place of titanium oxide, a photocatalytic apatite which itself has an adsorption ability. For example, calcium hydroxyapatite Ca10(PO4) 6 (OH)2 is a main component of living hard tissue such as teeth or bones and is readily ion-exchangeable with various cations or anions and accordingly, is assured of high biocompatibility and a high adsorption property. The calcium hydroxyapatite, particularly, has an ability of specifically adsorbing an organic material such as a protein. Actually, in contrast with a titanium oxide particle, a photocatalytic apatite can participate in adsorption and decomposition at all portions, and the decomposition efficiency does not decrease.
As for specific examples of the photocatalytic apatite, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-244166 discloses an environment cleaning material and, more specifically, a material for the removal of an offensive odor, the decomposition and removal of a harmful substance or contamination in air, waste-water treatment, water disinfection or the like, wherein a film comprising titanium oxide is formed on the surface of a substrate such as activated carbon, active alumina, silica gel, glass, foamed ceramic or foamed plastic, or a substrate comprising a titanium oxide particle is prepared, and a porous calcium phosphate (preferably hydroxyapatite, apatite fluoride or the like) is coated on the surface of the titanium oxide film or titanium oxide particle.
However, the titanium oxide and the photocatalytic apatite exhibit a problem common to these materials. That is, from the aspect of a antimicrobial property, these materials hardly show an antimicrobial property when disposed in a dark place (for example, in a state that ultraviolet light is not irradiated or ultraviolet light is not present). In the case of the photocatalytic apatite, even when exposed to ultraviolet light, only a slight antimicrobial effect can be recognized.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-156215 describes an antimicrobial agent (for example, silver apatite) comprising a metal ion-supporting crystalline ion exchanger in which a metal ion such as silver is supported on a support comprising a crystalline ion exchanger such as hydroxyapatite. This silver apatite can exhibit the antimicrobial property but does not have a photocatalytic function by itself and because of a lack in the photocatalytic activity, cannot be used for the removal of VOCs (volatile organic compounds) and the like. Not only the silver apatite but also generally known antimicrobial agents have no function of decomposing a virus and the like and along with accumulation of virus debris or the like during use, the original antimicrobial activity or the like cannot be exerted.